In traditional wireless networks for radio communication, such as GSM networks, a single narrowband frequency carrier is typically used for transferring data and messages in radio signals between the network and a wireless device connected to an access point of the network, either for transmitting downlink signals from the network to the wireless device or for transmitting uplink signals from the wireless device to the network. Recently, increasingly advanced user terminals and devices have emerged on the market, e.g. smartphones, tablets and wireless laptops, which are suitable for services such as internet browsing, streaming of media and any other service involving communication of large amounts of data. The demands for high data throughput over radio has therefore increased immensely.
In this description, a “wireless device” may be any communication equipment capable of communicating radio signals with an access point of a wireless network, such as a base station. The wireless device in this context could also be referred to as a mobile terminal, mobile station, User Equipment (UE), etc., depending on the terminology used. Further, a “base station” is an access point of a wireless network, arranged to communicate radio signals with wireless devices. The term “radio node” will be used herein to represent both a base station capable of transmitting downlink signals and a wireless device capable of transmitting uplink signals. It should be noted that the radio node in this description may further be capable of receiving radio signals as well, which is however somewhat outside the scope of this solution. Thus, a base station is typically also capable of receiving uplink signals and a wireless device is typically also capable of receiving downlink signals. In the context of radio communication, the term “carrier” is typically used which refers to a radio transmission on a certain frequency carrying information.
Traditionally, one carrier has been used for transferring information from a sending node to a receiving node. To meet the greater demands for data throughput, the possibility of using two or more carriers in parallel has been introduced such that the amount of data that can be communicated per time unit in either downlink or uplink, also referred to as data throughput, is basically multiplied by the number of carriers used. This feature thus introduces multiple parallel carriers transmitted on different frequencies to or from the same radio node, which thus means that a wider transmission bandwidth is used as compared to a single carrier which is transmitted within a narrower transmission bandwidth. The wider transmitter band may also be divided into several bands, so-called multi-band transmission such as carrier aggregation which has been introduced to enable higher data throughput.
Unwanted emissions may occur inside of a transmitter band and outside the transmitter band referred to as out-of-band emissions. Unwanted emissions may be caused by Inter-Modulation Distortion, IMD, causing spectral regrowth of transmission signal. Unwanted emissions outside a nominal transmitter band or frequency range are typically generated from the modulation process and non-linearity in the transmitter in the wireless device, whereas emission further away in frequency, i.e. spurious emissions, are typically caused by unwanted transmitter effects such as harmonics emission, parasitic emission, intermodulation products and frequency conversion products. These effects are typically reduced when the output power of the transmitter is reduced. Moreover, the wider the transmission bandwidth, the wider the unwanted spectral emissions.
Radio transmissions from a radio node using one or more carriers within a nominal transmitter band may therefore cause interference on a “neighboring” victim band located close to the transmitter band, since the radio transmissions also cause the above-mentioned emissions outside the nominal transmitter band. The victim band may for example be used for reception or transmission by another radio equipment in the radio node itself or by another node located nearby. In this description, the term neighboring victim band merely implies that the victim band is close to the transmitter band in frequency domain, i.e. close enough to be affected by interference from the above-described out-of-band emissions. In particular, Inter-Modulation Distortion, IMD, is generated by the power amplifier in the radio node. The interference caused by IMD may be harmful, e.g. by disturbing a receiver's reception in a receiver band thus being the victim band, such that sensitivity of the receiver is degraded. In this description, the term “victim band” represents any frequency band in which interference can be harmful and where it is desirable to avoid or at least reduce the interference caused by transmission in the transmitter band. Even though the described victim band is typically a receiver band, the description is not limited thereto.
For example, the transmitter band and the victim band may be used for transmission and reception, respectively, at the same node, e.g. at a base station or at a wireless device, as illustrated in FIG. 1 where a radio node 100 transmits radio signals in a transmitter band from a transmitter antenna 100a. This transmission includes IMD products outside the nominal transmitter band which are received by a receiver antenna 100b in a receiver band in the form of interference, as indicated by a dashed arrow. Alternatively, the same antenna may be used for both transmission and reception in a conventional manner. The above-described interference may also hit a co-located or adjacent node 102 where the transmitter antenna is more or less close to the receiver antenna, when generated IMD products are within the receiver band. In either case, it is desirable to avoid or at least reduce the above-described interference on the receiver band.
FIG. 2 illustrates that when a radio node transmits two radio signals S1 and S2 on different carriers using a power amplifier in a conventional manner, a number of IMD components are also generated by the power amplifier. The generated IMD components are denoted IMD3, IMD5 and IMD7 which are distributed symmetrically on either side of the nominal frequency band used for transmitting the signals S1 and S2. The power amplifier may also generate further IMD components of higher order depending on the amplifier's design but the following examples refer to the first three IMD components IMD3, IMD5 and IMD7 for simplicity. If the frequency of any of these IMD components coincides with a victim band such as a receiver band, they will cause interference on the victim band. This can be avoided or reduced by employing a duplex filter after the power amplifier which is capable of filtering away the harmful IMD components generated by the power amplifier, before the signals are emitted from the transmit antenna.
Another known way of handling IMD components is to apply Digital Pre-Distortion, DPD, on the signal before the power amplifier which effectively generates inverse distortion products at the frequencies of the IMD components, e.g. IMD3, IMD5 and IMD7, as illustrated by dashed arrows in FIG. 2, such that the inverse distortion components more or less cancel out the IMD components generated in the power amplifier. In other words, DPD generates IMD components with inverse amplitude and phase compared to the power amplifier generated IMD components such that the sum of the DPD generated IMD components and the power amplifier generated IMD components is minimized. DPD is an operation which uses a sampling technique in a well-known manner to generate the inverse distortion products. Briefly described, the DPD operation basically creates an inverse DPD model of the power amplifier's non-linearity and the inverse distortion products are created by applying the inverse DPD model to the signal to be transmitted. DPD is an example of a linearization technique for cancelling out IMD components while other linearization techniques such as Analog Pre-Distortion, APD, can also be used.
However, there are some problems associated with the above techniques for avoiding emission of harmful IMD components from the radio node. Firstly, the DPD technique may be feasible to employ when the transmitter band is fairly narrow, but if a wider transmitter band is employed the DPD technique is more difficult since more advanced equipment is needed for achieving a sufficiently high sampling rate in the DPD process in order to generate a DPD correction band that is wide enough to suppress the broader distribution of IMD components outside the nominal transmitter band. A high sampling rate also give rise to an increased power consumption. In particular, the DPD technique is normally employed in wireless devices only in a simplified way, if at all, due to the relatively high costs and high power consumption, and it would be even more costly to employ to allow two or more carriers to be transmitted.
Secondly, a relatively advanced duplex filter of high performance is required to filter away any IMD components generated by the power amplifier outside the nominal transmitter band, before the signals are emitted from the radio node's transmitting antenna. In conclusion, if multiple carriers are to be employed it is necessary to suppress a relatively broad distribution of IMD components outside the nominal transmitter band, which in turn requires the use of a linearization technique such as DPD with relatively high sampling rate and/or a duplex filter capable of filtering away the IMD components. It is thus a problem that the above-described DPD technique and the advanced duplex filter are costly and complex to employ, and may also result in high power consumption which may not be feasible e.g. in a wireless device.
FIGS. 3a and 3b are similar diagrams that illustrate schematically how the DPD correction band must be made wider if the nominal transmitter bandwidth is increased, e.g. by using a greater number of carriers for transmitting radio signals. In FIG. 3a, only one carrier is used which corresponds to a relatively narrow transmitter band C1. As a result, the IMD components IMD3, IMD5 and IMD7 generated by the transmission are relatively close to the band C1 in frequency domain. In order to avoid or reduce the interference on a neighboring victim band 302a, a relatively narrow DPD correction band 304a is sufficient to suppress e.g. the first two IMD components IMD3 and IMD5 on either side of the transmitter band C1, assuming that IMD7 is weak and not harmful in terms of interference in this example.
On the other hand, FIG. 3b illustrates another example when multiple carriers are used which correspond to a relatively wide transmitter band C2. As a result, the IMD components IMD3, IMD5 and IMD7 generated by the transmission becomes both stronger and further away from the band C2 in frequency domain. In order to avoid or reduce the interference on a neighboring victim band 302b, a much wider DPD correction band 304b is therefore required to suppress the IMD components IMD3, IMD5 and IMD7, in case all of them are strong enough to be harmful in terms of interference. As explained above, it is both costly and power consuming to achieve such a wide DPD correction band at the radio node.